52 



AGRICULTURAL ENGINEERING 



instrument and thus obtain the elevation of several points 

 between A and B. This is done in practice. 



It is to be noted in this connection that back sights are 

 rod readings on stations or points whose elevations are known, 

 and fore sights are readings on stations whose elevations are 

 ot known. Stations on which back sights are taken are 

 generally known as turning points. 



Stakes. It is generally best that all stations be marked 

 with a stake driven down close to the ground, on which the 



leveling rod may be placed; 

 and turning points should 

 always be so marked and 

 identified. 



Leveling a Field. It is 

 sometimes advisable to obtain 

 levels at regular intervals 

 over an entire field. This is 

 accomplished by laying the 

 field off into squares, usually 

 by the chain or tape. The 

 corners of the squares are 

 marked with stakes made of 

 lath and the elevation of the 

 top of the ground is taken at each corner, as shown in Fig. 39. 

 The various corners of the squares are designated by lettering in 

 one direction and numbering in the other as shown in the figure. 

 Contour Maps. Lines may be drawn over the map of 

 the leveled field to indicate points of equal elevation. Such 

 lines are called contour lines. They offer a very satisfactory 

 means of studying the surface of the ground, and a map so 

 prepared is especially useful in laying out drainage systems. 

 Horizontal Circles for Levels. Many levels are pro- 

 vided with horizontal circles or scales, graduated in degrees 



Fig. 39. Plat showing how levels 

 may be taken over an entire field. 

 The stations are indicated by letter 

 and numbers, as Bs, etc. 



